In general, as fuel cells, there are a phosphoric acid fuel cell (PAFC), an alkaline fuel cell (AFC), a polymer electrolyte fuel cell (PEFC), and the like. Among them, in comparison with the other fuel cells, the polymer electrolyte fuel cell (PEFC) can be activated at normal temperature, has fewer problems on dissipation and holding of an electrolyte, and is easy to maintain. However, the polymer electrolyte fuel cell has problems that it is necessary to precisely manage moisture in an electrolyte membrane thereof, and the like. Such management of the moisture in the electrolyte membrane is an important subject, and it is essential to manage the moisture in the electrolyte membrane in order that the electrolyte membrane can have good proton conductivity in a state of containing the moisture therein. Accordingly, in general, the electrolyte membrane is kept in such a moisture-containing state by means of a humidifier and the like. Here, when a thickness of the electrolyte membrane is thin, it is easy for the electrolyte membrane to maintain the moisture-containing state, and the electrolyte membrane can suppress a membrane resistance thereof. On the other hand, the thin electrolyte membrane has had a problem that a mechanical strength thereof is decreased.
As a technology for solving such a problem, a hydrocarbon electrolyte membrane into which filler is dispersed is disclosed in Japanese Patent Unexamined Publication No. 2005-222736. In this electrolyte membrane, a high strength is brought up by the filler, and swelling is suppressed in the entirety thereof. Accordingly, a stress in each cell is decreased, whereby mechanical durability of the electrolyte membrane in the fuel cell is enhanced.
Incidentally, the electrolyte membrane of the fuel cell shrinks in a dry environment, and swells in a wet environment. In the fuel cell, an electrode catalyst layer is coated on the electrolyte membrane. Then, the electrolyte membrane is assembled together with a gas diffusion layer and a separator having a flow passage that flows gas therethrough and having a rib that conducts electrons therethrough. Finally, these components which are the electrolyte membrane, the gas diffusion layer and the separator are stacked on one another at a surface pressure of 0.1 MPa to 2 MPa. Therefore, the electrolyte membrane is operated while the entire surface thereof is being dynamically restricted. Under such conditions where the electrolyte membrane is used, the electrolyte membrane cannot freely swell or shrink following that it is dried or humidified as being operated. Accordingly, a swelling stress or a shrinking stress occurs. Moreover, with regard to both of the swelling stress and the shrinking stress, local concentration thereof is caused by a distribution of a compressive stress. Here, the distribution of the compressive stress is caused by a fine step difference, pinhole and surface roughness of the electrode catalyst layer coated on the surface of the electrolyte membrane, by a thickness distribution of the gas diffusion layer, by a shape of the flow passage of the separator, or by a shape of the rib. When the electrolyte membrane repeatedly swells and shrinks in such an environment of being applied with the stress, a crack occurs in the electrolyte membrane. A progress of the crack finally results in a fracture of the electrolyte membrane.